1. Field of the Invention
The present invention relates to blowing agents for the production of foams and methods for the use of these blowing agents, and more particularly to silicate-based blowing agents for the production of foamed polymers and methods for the use of these silicate-based blowing agents.
2. Description of the Related Art
The use of blowing agents to create foamed polymers is well known. (See, e.g., Modern Plastics Encyclopedia, 57 (10A): 200-203, 214-221, 300-310, October 1980, McGraw-Hill Inc., New York, N.Y., which pages are incorporated here in by reference). Blowing agents are usually either gases that are dispersed throughout the polymer by high shear mixing or by injection under pressure, or are liquids or solids that are dispersed throughout the polymer and generate gas by chemical decomposition or evaporation. All blowing agents result in the formation of gas-filled cells throughout the polymer. The cells result in the formation of a sponge or foam structure that has a lower bulk density than the solid polymer.
Blowing agents that generate gas after their incorporation into the polymer are termed "in situ" blowing agents. They function by chemical decomposition of the blowing agent or because of a phase change from a solid or liquid to a gas phase under the conditions of the blowing step. An advantage of in situ blowing agents is that they do not require the energy-intensive step of incorporating gas into a polymer by high shear mixing or by high pressure injection. Another advantage is that foaming can be initiated after injecting the polymer into a mold so that molded foamed products can be produced.
Chlorofluorocarbons (CFC's) are commonly used as in situ blowing agents for the manufacture of synthetic foams. Likewise, azo- compounds, such as 1-1' azobisformamide (ABFA) or azide compounds, such as 4,4'-oxybis (benzenesulfonylhydrazide), which generate nitrogen upon chemical decomposition, are often used in foamed rubber manufacture. Both CFC's and the nitrogen-forming azo- and azide compounds, however, have disadvantages. CFC's have been identified as participating in the destruction of the earth's ozone layer and azo- and azide blowing agents can form nitrosamines, which have been shown to cause unwanted health effects. Furthermore, conventional azo- and azide blowing agents also require a relatively high blowing temperature and are often required in relatively high concentrations to obtain a given degree of foam expansion--especially with foamed rubbers.
One alternative to CFC's and nitrogen-forming blowing agents is the use of silicate-based materials. The use of silicates to form rigid foams is well known and has been reported in, for example, U.S. Pat. Nos. 3,933,514, 3,961,972 and 4,848,465.
Rigid, open cell silicate foams having good compressive strength were reported in U.S. Pat. No. 5,242,494, to Callaghan et al. The foamable composition contained at least 20% by weight of an metal silicate (and preferably 35% to 40%), a blowing agent (which could be hydrogen peroxide), a surfactant and a hardener that was capable of liberating acetic or formic acid under the conditions of foaming. It was reported to be advantageous to include a water-dispersible polymer in the foamable composition under certain circumstances, however, no rubbers were said to be useable and only rigid foams were produced.
Summers et al., in U.S. Pat. No. 4,057,519, disclosed a rigid foam with improved flame retardance at a lower cost than conventional flame retardant additives. The improvement was obtained by the inclusion of aqueous sodium silicate in a composition with an hydroxyl-terminated polyester, a polyisocyanate prepolymer, a halogenated alkane blowing agent and a catalyst, such as tin, an amine, or paratoluenesulfonic acid.
In U.S. Pat. Nos. 5,246,654 and 5,501,826, Ertle et al. describe the production of dense, free-flowing alkali metal silicate-based particles, which, when heated to a temperature of from about 250.degree. to 1100.degree. F., expand to form rigid, lightweight foamed particles. The particles are formed from liquid sodium or potassium silicate with the addition of some combination of magnesium silicate, calcium carbonate and/or boric acid or sodium borate pentahydrate, or Portland cement. The expanded rigid particles are said to be useful for thermal or acoustical insulators, as well as for bulking agents for concrete or gypsum.
More recently, Ertle et al., in U.S. Pat. No. 5,612,386 (which is incorporated by reference herein), have disclosed the use of silicate compositions similar to those described above as blowing agents for thermoplastic and thermoset polymers, or as accelerators or initiators for other organic blowing agents. The blowing agents were prepared by mixing an alkali metal silicate solution with hydrous magnesium silicate and boric acid solution and evaporating water until a solid material was obtained. The solid material was comminuted and further dried to form the blowing agent.
As Ertle et al. explain, in the production of a foamed thermally processable polymer, such as rubber, the polymer will only capture and hold gas bubbles during a relatively short interval in its processing. Gas is captured after the polymer has cured sufficiently to provide the necessary film strength and viscosity required for gas capture, and gas capture continues until the film strength and viscosity exceed a limit where the cells will rupture and can no longer hold gas. But gas that is generated by the blowing agent before the polymer has entered, or after the polymer has passed through, the gas capture window is lost. The period of time in which gas is captured within the polymer to form a foam is termed the "gas capture window". Because azide blowing agents generate gas at a slow and steadily increasing rate, much of the gas that they are capable of generating is produced either before or after the gas capture window. This has been cited as a reason why a relatively high concentration of these azides is needed to achieve a given amount of cell volume. (See, e.g., Cylacell, Endothermic Blowing Agent, informational product literature from Cylatec, 3711 Whipple Ave., N.W., Canton, Ohio 44718). This phenomena results in a reduction in blowing efficiency. As used herein, the term "blowing efficiency" means the amount of the gas generated by a given concentration of a blowing agent that is captured within the cells of the foam relative to the total amount of gas that is generated by the blowing agent.
An advantage that is described for one silicate-based blowing agent is that it generates gas within a much narrower time and temperature window than is typical for azides. (See, e.g., the Cylacell.TM. product literature referenced above). When the conditions for gas generation by the silicate-based blowing agent are adjusted to coincide with the gas capture window of a polymer, it is claimed that the silicate-based blowing agent exhibits a blowing efficiency that is several times higher than typical azo- or azide blowing agents. However, in order to obtain this increased efficiency, the gas generation peak of the blowing agent must be adjusted to coincide with the gas capture window of the polymer. Since this matching requires adjustment of blowing conditions and the composition of the blowing stock, it often requires trial and error testing. Gas generation that occurs either before or after the gas capture window of the polymer could result in very low gas capture, yielding an unsatisfactory foam. Since it is common for many additives to be blended with a polymer during compounding, it would not unusual for the gas capture window of polymers to vary somewhat from batch to batch. Such variation could cause either inconsistent gas capture efficiency in silicate-based blowing agents having a narrow gassing period, or could require an untoward amount of trial and error testing. This problem may be especially noticeable in polymers having a limited gas capture window, such as many rubbers.
Accordingly, it would be useful to provide a blowing agent for polymers, especially for rubbers, that gave the same bulk density of the foamed polymer as conventional azo- and azide blowing agents, but at lower levels of use. It would be useful if this blowing agent were also easily dispersible throughout the polymers during compounding. Moreover, it would also be useful if this blowing agent had a lower activation temperature than conventional azo- and azide blowing agents, and known silicate-based blowing agents, and also demonstrated consistent blowing efficiency, especially when used to produce foamed rubbers.